Abstract

We report the measurement of the essentially frequency independent refractivity of water vapor from 0.1 to 1 THz, independent of the simultaneous strong THz pulse broadening and absorption. The humidity dependent transit time of THz pulses through a 170 m long round trip path was measured to a precision of 0.1 ps, using a mode-locked laser as an optical clock.

A. Deepak, T. D. Wilkerson, and L. H. Ruhnke, eds., Atmospheric Water Vapor, (Academic Press, 1980). This book is the Proceedings of the International Workshop on Atmospheric Water Vapor, Vail, Colorado, September 11–13, 1979.

D. E. Burch and D. A. Gryvnak, “Continuum absorption by water vapor in the infrared and millimeter regions,” Proceedings of the International Workshop on Atmospheric Water Vapor, [21] 47–76 (1979).

Other (6)

A. Deepak, T. D. Wilkerson, and L. H. Ruhnke, eds., Atmospheric Water Vapor, (Academic Press, 1980). This book is the Proceedings of the International Workshop on Atmospheric Water Vapor, Vail, Colorado, September 11–13, 1979.

D. E. Burch and D. A. Gryvnak, “Continuum absorption by water vapor in the infrared and millimeter regions,” Proceedings of the International Workshop on Atmospheric Water Vapor, [21] 47–76 (1979).

(a) The measured pulse time-shift vs RH at 21 °C for four independent measurements, performed many weeks apart. (b) Replot of the data and extended lines of (a), with the time delay at zero RH subtracted from the data series. The overlap of all data was fit a straight line with a standard deviation of 1.40; and a slope: 5.84 ps/ RH 10%. For conversion to other units RH 54% at 21 °C is equivalent to 10.1 mm Hg and a water vapor density of 10 g/m3.

Total refractivity of water vapor at 20 °C and 10 g/m3. (a). The calculated refractivity of all the water vapor lines (Fig. 3(b) [12]), added to the measured frequency-independent refractivity (yellow highlight) of (70 x 10−6)/[1 + (f/f1/2)2] for f1/2 = 2 THz. (b) The calculated refractivity of all the water vapor lines [12], added to the adjusted frequency-independent refractivity (yellow highlight) of (61 x 10−6)/[1 + (f/f1/2)2] for f1/2 = 2 THz. The measurements of [1–3] are indicated by the three open circles at 61 x 10−6 on the dashed lines at 9.2, 24 and 72 GHz, respectively. The measurements of [5] are indicated by the two open circles at 63 and 80 x 10−6 on the dashed lines at 0.890 and 0.965 THz. It should be noted that without the frequency-independent term (n(0) – 1)/[1 + (f/f1/2)2 ], the curves drop to the zero base line, and become identical to Fig. 3(b) [12].